LAUSR

Optic nerve sheath diameter (ONSD) is associated with poor neurological outcomes following resuscitation. 2 However, the cutoff values of ONSD vary and are similar to those of normal adults. 3 The ratio of ONSD to eyeball transverse diameter (ETD) would be a better indicator for intracranial hypertension. However, the performance of the ONSD/ETD ratio in cardiac arrest survivors has not been studied. We conducted a prospective agematched and sexmatched nested case–control study to investigate the performance of sonographic ONSD and ONSD/ETD ratio between outofhospital cardiac arrest (OHCA) survivors with good and poor outcomes compared with healthy controls. Consecutive adult nontraumatic OHCA survivors between March 2020 and February 2021 and healthy controls were prospectively enrolled. Those with prior ophthalmological operations and a history of a brain tumour or brain surgery were excluded. Each OHCA survivor was matched to four controls by age and sex. This study was registered at ClinicalTrials. gov (NCT04510363). The sonographic examinations were performed at 24, 72 and 168 hours postreturn of spontaneous circulation (ROSC) (for survivors) or study enrolment (controls). Three measurements were taken at each time point and averaged. The ONSD was measured between the outer hyperechoic borders of the optic nerve sheath at 3 mm posterior to the globe using a 7–12 MHz linear transducer (Xario 100, Canon, Tokyo, Japan). The ETD was the maximal transverse diameter from the retina to the retina in the horizontal plane (figure 1). The mechanical index was set as less than 0.3. Sonographers were emergency physicians who attended advanced emergency ultrasound training certified by the Taiwan Society of Emergency Medicine. Neurological outcomes were assessed at 168 hours postROSC by intensivists blinded to the sonographic results. Poor neurological outcome was defined as a GlasgowPittsburgh Cerebral Performance Category score of 3–5. All data were analysed using SAS software (V.9.4). Categorical data were compared using a Χ test or the Fisher’s exact test. Continuous data are expressed as medians and IQRs and were analysed using the Wilcoxon ranksum test. A repeatedmeasures analysis using a mixed regression model for ONSD and the ONSD/ETD ratio was performed to account for timevarying effects. Covariates included medical history, witness arrest, bystander cardiopulmonary resuscitation, initial shockable rhythm and cardiogenic arrest. A p value of <0.05 was considered statistically significant. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for the optic sheath measurements were computed as well as the area under the receiver operating characteristic (AUROC). A total of 30 OHCA survivors were included with a median age was 67 years and 63% male (table 1, online supplemental table 1, and online supplemental figure 1). No timevarying effects of ONSD and ONSD/ETD ratios were noted in mixed regression models. Twenty (67%) patients had poor neurological outcomes; the ONSD (6.4 mm vs 5.8 mm, p<0.001) and the ONSD/ETD ratio (0.28 vs 0.25, p<0.001) at the 24hour measurement were significantly higher than the normal controls. Those with good neurological outcomes had a similar ONSD (5.6 mm vs 6.0 mm, p=0.394) and ONSD/ETD ratio (0.24 vs 0.26, p=0.112) to the controls. The ONSD and ONSD/ETD ratios had similar AUROCs (online supplemental figure 2). The ONSD of 6.2 mm displayed a sensitivity of 66.7% (95% CI 44.9% to 88.4%), specificity of 90.0% (95% CI 83.4% to 96.6%), PPV of 60.0% (95% CI 38.5% to 81.5%) and NPV of 92.3% (95% CI 86.4% to 98.2%). The ONSD/ ETD ratio of 0.28 exhibited a sensitivity of 44.4% (95% CI 21.5% to 67.4%), specificity of 93.8% (95% CI 88.5% to 99.1%), PPV of 61.5% (95% CI 35.1% to 88.0%) and NPV of 89.2% (95% CI 82.5% to 95.9%) (online supplemental table 2). There were limitations in this study. The data were obtained from a single centre in which the OHCA survivors had relatively lower rates of initial shockable rhythm and cardiogenic arrest. However, the ONSD and ONSD/ETD ratios remained significant after adjusting the initial rhythm. Second, sonographic measurements would be operator dependent. Third, Research letter